1. Field of the Invention
The present invention is directed to a method for reconstructing a CT image, and in particular to reconstructing a CT image using a reconstruction algorithm for a short-scan circle combined with various lines.
2. Description of the Prior Art
Tomographic imaging of high contrast objects based on cone-beam projections acquired on C-arm systems as described, for example, in M. Grass, R. Koppe, E. Klotz, R. Proksa, M. Kuhn, H. Aerts, J. O. de Beck, and R. Kempkers, “Three-dimensional reconstruction of high-contrast objects using C-arm image intensifier projection data,” Comp. Med. Imag. and Graphics 23, pp. 311-321, 1999 has become established in a clinical, interventional environment. In particular in neuroradiology the 3D representation of the complex vascular tree is of high clinical value to plan or validate therapy. Due to the invasive, arterial injection of contrast agent the vascular tree possesses a much higher contrast to the surrounding tissue such as e.g. bone. Thus, the procedure is relatively insensitive to distortions and image artifacts. Recent improvements in data acquisition, e.g. by use of flat panel detectors, will shift clinical applications towards imaging of low-contrast objects. For example, diagnosis and treatment of stroke on the same C-arm device is a highly desirable goal. This would require that hemorrhage in brain matter be ruled out before treating ischemia. According to current clinical protocols this is done by a native computed tomography (CT) scan. Soft tissue imaging requires accurate data acquisition and processing as described in M. Zellerhoff, B. Scholz, E.-P. Ruehrnschopf, and T. Brunner, “Low contrast 3D-reconstruction from C-arm data,” in Proc. SPIE 5745, to be published, 2005 and J. Wiegert, M. Bertram, D. Schaefer, N. Conrads, N. Noordhoek, K. de Jong, T. Aach, and G. Rose, “Soft tissue contrast resolution within the head of human cadaver by means of flat detector based cone-beam CT,” in Proc. SPIE 5368, pp. 330-337, 2004. A serious limitation is the incompleteness of projection data acquired by a conventional short-scan circular source trajectory. Cone artifacts, which result from that incompleteness, occur as a smearing and shading artifact and may superpose severely important low contrast details.
Numerous investigations on source trajectories that satisfy Tuy's completeness condition (see H. K. Tuy, “An inversion formula for cone-beam reconstruction,” in SIAM J. Appl. Math, 1983) can be found in the literature: saddle trajectory (J. Pack, F. Noo, and H. Kudo, “Investigation of saddle trajectories for cardiac CT imaging in cone-beam geometry,” Phys. Med. Biol. 49, pp. 2317-2336, 2004) selection of non-planar, non-closed trajectories optimized for C-arm devices, (H. Schomberg, “Complete source trajectories for C-arm systems and a method for coping with truncated cone-beam projections,” in Proc. Meeting on Fully 3-D Image Reconstruction in Radiology and Nucl. Med., 2001) circle and arc trajectory optimized for CT gantries, (R. Ning, X. Tang, D. Conover, and R. Yu, “Flat panel detector-based cone beam computed tomography with a circle-plus-two-arcs data acquisition orbit: Preliminary phantom study,” Med. Phys. 30, pp. 1694-1705, 2003) circle and line trajectory (G. L. Zeng and G. T. Gullberg, “A cone-beam tomography algorithm for orthogonal circle-and-line orbit,” Phys. Med. Biol. 37, pp. 563-577, 1992, and R. Johnson, H. Hu, S. Haworth, P. Cho, C. Dawson, and J. Linehan, “Feldkamp and circle-and-line cone-beam reconstruction for 3D micro-CT of vascular networks,” Phys. Med. Biol. 43, pp. 929-940, 1998 and H. Kudo and T. Saito, “Fast and stable cone-beam filtered back-projection method for non-planar orbits,” Phys. Med. Biol. 43, pp. 747-760, 1998) and many others.